So, I thought about this a little and here's my take:

Mark is a grown-up version of Juno. Just like Juno, he's into all that great counter culture stuff: the music, the horror flicks and what not. And with all the hipster culture stuff, he's taken on an hip ironic detachment as well, again just like Juno. Mark has used this ironic detachment to avoid confronting the reality that he is now an adult with real obligations and emotional attachments. So when it comes time for him to really step up, he freaks out and bails. It doesn't help much that Juno reminds him of his teenage self.

When Juno sees this happen, she realizes her same breezy attitude toward life may lead her down the same path as Mark. So she decides to veer off that path by going through with the adoption and acting on her feelings for Bleeker.

That's my simplistic take anyway.

Thanks for your insights Mullac, some of them were similar to mine, although I didn't think of Mark as a warning for Juno until you pointed out.

Many of the guys I know are late twenty-something, early thirties dudes who are in bands, and desperately trying to "make it." Nearly all of them are unmarried and may or may not have been in long term relationships.

There take has been that Mark's wife was a shrew who was snuffing Mark's dream and that Mark was a sympathetic character.

My take, colored by my own experience (I was in bands until a year ago, and now have a pregnant wife) is that while Mark's character starts out as the sympathetic one, after Juno sees Vanessa being a good parent in the mall (by herself no less) that the tables shift a bit.

Mark is stuck in trying to achieve a dream he already had a shot at (toured Japan?) and won't grow up.

While Vanessa seems initially a bit crazy for desperately wanting to be a mother, it's a much more realistic dream than Mark's which is some intangible rock lifestyle.

Yeah, the movie definitely sets up the viewer to sympathize for Mark and detest Vanessa and then does the ol' switch-a-roo. It definitely worked on me.

Though I don't think Mark's problem is that he has unrealistic dreams or that he wants to pursue some sort of rock'n'roll lifestyle--his problem is that he doesn't have the guts to tell that to his wife before he took on commitments he wasn't serious about. He wants it both ways: the rock'n'roll lifestyle and the comfort of a typical marriage/home/job combo. Clearly this is an untenable situation and he's a bit of a dick for not owning up to the commitments he made. Vanessa may be a bit of a shrew, but she's consistent in her goals and doesn't deserve what Mark does to her.

I suppose I'm sorta like your friends. I'm not in a band, but I'm coming up on my late 20s, never been in a long-term relationship and have a career that isn't compatible with "settling down" (at least, in a way that I'd want to do it). But I'm fine with putting off having a family until my situation changes and I'm ready to handle that kind of commitment (both emotionally and time-wise).

On my first and only viewing, that part kind of took me out of the movie for a bit. I was just like "Wait. What? What the hell?" and couldn't really wrap my brain around it. It didn't seem like it fit the rest of the movie to me. But mullac's interpretation makes a lot of sense to me, so we'll see if I feel the same when I see it on DVD.

mullacc, I like your explanation, but I still feel like Jason Bateman's character was flat. At first you think, "Yeah, he's the cool guy! This is the reason Juno is okay with giving her baby to these crazy folks." And then he turns out to be another jerk with a Peter Pan complex. But he doesn't redeem himself in any way, he doesn't grow, and I feel like just about every other character does.

Admission: I had a crush on Jason Bateman when I was a kid, and I must admit he's still looking pretty cute to me. Am I insane???